refs: update ref transactions to use struct object_id

Update the ref transaction code to use struct object_id.  Remove one
NULL pointer check which was previously inserted around a dereference;
since we now pass a pointer to struct object_id directly through, the
code we're calling handles this for us.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/replace.c b/builtin/replace.c
index 2854eaa..3099e55 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -175,7 +175,7 @@ static int replace_object_oid(const char *object_ref,
 
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, ref.buf, repl->hash, prev.hash,
+	    ref_transaction_update(transaction, ref.buf, repl, &prev,
 				   0, NULL, &err) ||
 	    ref_transaction_commit(transaction, &err))
 		die("%s", err.buf);